Recently there is provided a CRT display system having a CRT display unit which provides displays with a CRT and other devices (such as a display unit other than a CRT and a video recorder) such as a television receiver incorporating a VCR therein. In the CRT display system described above, a spot killer operation is executed with a spot killer circuit to prevent spots from remaining on a screen when power is turned OFF.
In the television receiver incorporating a VCR therein, when images are recorded with a VCR while displays are being provided by the CRT, sometimes it is desired to delete a screen of the CRT without stopping the operation for recording video images. When a screen is to be deleted while recording video images, it is necessary to set only the CRT in the stand-by state when power is ON. Generally in order to set a CRT of a CRT display unit in the stand-by state when power is ON, sweeping in the horizontal or vertical direction is stopped, and generation of a high voltage in a flyback transformer as a source of a voltage for accelerating electrons within the CRT is stopped. However, there is the problem that, after the CRT is set in the stand-by state, spots remain on the screen until a high voltage in the CRT accumulated in the capacitor disappears due to discharging.
As the conventional type of CRT display system which can solve the problem described above, there is the one which executes a spot killer operation for neutralizing a high voltage in the CRT by flowing a current in the CRT using an OSD (On Screen Display) in a state where power is ON. FIG. 5 shows general configuration of the CRT display system.
This CRT display system comprises a CRT 31 providing displays, a deflection yoke 32 which sweeps an electron beam in the CRT 31, a sawtooth wave generation/output circuit 33 which generates a sawtooth wave for sweeping the electron beam and outputs the sawtooth wave to the deflection yoke 32, a deflection signal processing circuit 34 which receives a horizontal/vertical synchronization signal, generates a horizontal/vertical timing pulse for generation of a sawtooth wave (sawtooth wave generation timing pulse), and outputs the sawtooth wave generation timing pulse to the sawtooth wave generation/output circuit 33, a video signal processing circuit 35 which receives a video signal and converts the video signal to R/G/B signals, an OSD 36 which generates signals such as channel displays and executes switching between the R/G/B signals from the video signal processing circuit 35 and signals such as channel displays, a switch 37 which stops supply of the R/G/B signals, a spot killer circuit 38 which generates a pulse-like white signal based on the R/G/B signals when power is turned ON, an R/G/B output circuit 39 which receives the R/G/B signals from the spot killer circuit :38 and drives a cathode of the CRT 31, and a microcomputer 40 which stops supply of a sawtooth wave generation timing pulse by controlling the deflection signal processing circuit 34, stops supply of the R/G/B signals by controlling the switch 37, and executes a stop killer operation by controlling the OSD device 36.
In this CRT display system, when it is required to set the CRT 31 in the stand-by state in the power OFF state, the microcomputer 40 performs following operation. That is, (1) output white on all screens with the OSD device 36, (2) stop supply of a sawtooth wave generation timing pulse with the deflection signal processing circuit 34, and (3) stop supply of the R/G/B signals with the switch 37. When the operation (1) is executed, a maximum quantity of current is supplied from a cathode into the CRT 31, and when the operation (2) is executed, generation of a high voltage in a flyback transformer is stopped. When the operation (1) is executed, the current supplied from the cathode to the CRT 31 flows neutralizing a high voltage in the CRT as a source for the current itself, thus an equilibrium being achieved in the CRT (spot killer operation). Finally when the operation (3) is executed, supply of a current from the cathode is stopped. With this operation, a voltage loaded to the CRT 31 disappears with no spot generated on a screen of the CRT 31, and the CRT 31 is set in the stand-by state.
Further, in this CRT display system, when power is turned ON, the spot killer circuit 38 generates a one-shot pulse. Therefore, a current is supplied into the CRT 31 and flows neutralizing a high voltage in the CRT as a source for the current itself, thus an equilibrium being achieved in the CRT with generation of spots on a screen of the CRT prevented.
With the conventional technology as described above, however, the spot killer circuit 38 is used only when power is turned OFF. Therefore, when it is required to, set the CRT 31 in the stand-by state in the power ON state, a spot killer operation is executed with the OSD device 36. Thus, when a function such as the OSD function for outputting white on all screens under control by a microcomputer is not available, it is necessary to provide a power source unit for the spot killer circuit 38 separately from and in addition to a power source unit for the circuit (for the VCR or the like) having other functions and to cut turned OFF only power for the spot killer circuit 38. Accordingly, there occurs the problem that the circuit becomes complicated and the cost disadvantageously increases.
Further, an operation for generation of a signal for the spot killer operation is executed using the spot killer circuit 38 when power is turned OFF, and using the OSD device 36 when power is ON, which is disadvantageously redundant. Furthermore, control signals for the deflection signal processing circuit 34, switch 37, and OSD device 36 are different from each other, which makes the processing with a microcomputer for the spot killer operation complicated and also causes increase in size of the control program and cost.